We're constantly being told to do what's good for us but it can be difficult to know what's good for us when the goal posts keep moving.

When it comes to exercise, the first real recommended guidelines originated from the American College of Sports Medicine back in 1978. Since then their ongoing research has provided the blueprint for much of the rest of the world.

One of the big differences between their recommendations in 1978 and more recent times has been the shift in emphasis from fitness to health.

This is because general physical activity was not an issue in the late 70s, as normal days contained more activity, with manual labour a big provider of employment; cars were in short supply, so people walked everywhere; and TV and Facebook were not occupying everyone's spare time.

As people have become less active in the intervening years, obesity has reached epidemic proportions and the general guidelines and recommendations have had to continually evolve to try and counteract this.

In 1978 the American College of Sports Medicine (ACSM) settled on a set of basic guidelines that became known as the FITT principles: frequency, intensity, time or duration and the type or the mode of activity. All subsequent recommendations have evolved from this.

Frequency: The one area where the ACSM has remained constant through the years is with regard to frequency, saying three to five days exercise per week is required. This is based on the fact that improvement in fitness gains plateaus at three times per week, while two training sessions is not enough.

Intensity: The intensity of training is measured in terms of what is known as V02 max. V02 max measures fitness by the volume of oxygen you can consume while exercising at your maximum capacity. Those who are fit have higher VO2 max values and can exercise more intensely than those who are not as well conditioned.

The guidelines remained the same until 1998 when the ACSM reduced the V02 max recommendation to 40pc, from 60pc.

In 1990, the impact that medication can have on heart rate was recognised as people on beta blockers (drugs used to manage heart conditions) have lower heart rates, which will therefore affect their fitness programmes.

Time: Initial recommendations from the ACSM indicated that 15-60 minutes of continuous aerobic activity, depending on intensity, was required. As time and knowledge improved, health benefits were found from exercising for just 10 minutes a day.

Type: In 1978, the mode of recommended activity was purely aerobic endurance training. In fact, these early guidelines left out any information on resistance training.

This is because, at the time, the whole genre of weight training was still considered quite underground and therefore precious few studies were done on it, therefore there was little knowledge of its benefits.

By 1990 this had changed and the ACSM recommendations included guidelines for resistance training. At this point they also made the distinction between the effect of low reps and high reps on performance strength.

Strength is developed by performing few repetitions and endurance is enhanced by performing more repetitions.

In addition they acknowledged that a full range of movement had the greatest benefit and that increasing the weight, repetitions or length of work-out over your baseline was critical for progression.

This is the Principle of Overload. When you train, your body must receive a stimulus to change otherwise it will stay the same.

Some of the most recent research done in this area has found that the correct application of resistance training can have many of the same benefits attributed to cardiovascular training: it can lower blood pressure, improve glucose metabolism and reduce cardiovascular disease.

Effects

The different effects of training on men and women are also now known. Women have less blood volume, fewer red blood cells, and less haemoglobin, leading to a lower oxygen-carrying capacity -- knowledge that was all lacking back in 1978.

Meanwhile, in older populations -- an area where knowledge was also previously lacking -- resistance training has now been shown to increase their quality of life, specifically in relation to reducing the risk of falling.

Here in Ireland, our current national guidelines on physical activity are 30 minutes of moderate exercise five times per week for those aged 18-64. However this is intended as a way of 'beginning the process' towards long-term health and fitness and comes from the point of view that any level of activity is better than none.

Major changes over the years have brought us to where we are today. The fact of the matter is that an unfit population has far-reaching consequences. Physical activity reduces blood pressure, hypertension, risk of cardiovascular diseases and some forms of cancers.

By doing a little bit of exercise every day and gradually increasing the frequency, intensity, duration and by varying between resistance training and cardiovascular training, you can improve your quality of life.

Your goal should always be to increase your daily activities whatever they may be. It may be challenging, especially if you have been out of commission for a long time. But you're never too early or too late to start and when you start, keep going and be fit for life not just for the days coming up to Christmas. Your body will thank you for it.